What does abbreviations like DEF, MCH, CTRL, WGT stands for in rigging?
Every rigger uses own bone naming conventions, prefixes or suffixes, usually in agreement with animators for which the rig is intended (in studio). There is no standard, but there are however some commonly used terms:


ROOT, MASTER - first bone in hierarchy or sub-hierarchy (tentacle_root)
DEF - bone contributing to deformation
MCH - mechanism/mechanical helper bone
ORG - original bones of metarigs from which the final rig is generated
CTRL, CON - control bones - various bones meant to be operated by animators
JNT - a joint
WGT, SHP, CS - widget, shape, custom shape, etc. - meshes to represent bones
IK, FK - bones belonging to inverse or forward kinematic bone chains
STR, NSTR, ASTR - stretch bones, non-stretch bones, anti-stretch bones (I've also seen SCA, NSCA, ASCA as scale, non-scale and anti-scale)
SPL - IK spline bones
MM, MUSC - muscle mass, muscle - bones to represent and bind muscles
POLE - IK pole bones, also usually just marked as CTRL
CW, CCW - clock-wise, counter-clock-wise, for marking rotation direction or bones for hinges
CEN, LFT, RGT, LF, RT, L, R - center, left, right